The biggest issue to look out for is making sure that all of your QVD's load in Optimized (super fast) mode. This will not be the case if all QVDs loaded do not have exactly the same columns in them. Any translation of the data on the way out of the QVD will cause it not to load in Optimized mode - search communities for more information on this.
What I would suggest you do in this instance is to add the extra thirty columns to the old QVDs with nulls or zeros (use zeros for numerics as QlikView is more efficient summing zeros than ignoring nulls). If you can run the creation routine for the old QVDs again it is straightforward to add the new columns at this point. If that is not possible you can create a QVD generate routine that loads from the old QVD's with the extra columns added and then writes new QVD's out again. This second approach will undoubtably be quicker than going back to the original source.
Hope that helps - please post back if you have further questions.